Exam Study Guide: Dissolved Oxygen Levels; and Legionella Removal Requirements

Maintaining your education is important, especially in a career that demands licensing exams. Prove you’re an expert operator by answering these questions and others from our Exam Study Guide Series.

Welcome back to TPO magazine's Exam Study Guide Series, which offers a pair of water/wastewater study questions with in-depth explanations of the answers. Last time, we covered a set of wastewater and drinking water treatment questions on the topics of Preliminary Treatment Safety; and Sulfuric Acid Titration. This time, you can test your knowledge about dissolved oxygen levels, and Legionella removal requirements.

Wastewater Treatment Sample Question

What is the recommended dissolved oxygen level in the aeration basin in a typical activated sludge facility?

A. Less than 0.5 mg/l
B. 0.5 to 1.0 mg/l
C. 1.0 to 2.0 mg/l
D. 4.0 mg/l or higher

Answer: The answer to this question is C. Dissolved oxygen is a key requirement for the proper operation of an activated sludge process to maintain aerobic conditions. Typical activated sludge processes run very well in the 1.0 to 2.0 mg/l DO range. The DO levels should be continuously monitored, due to factors such as water temperature or wastewater quality that may degrade the oxygen requirements of the microorganisms. Higher DO levels (over 4 mg/l) rarely helps the process and effectively wastes electric costs. Some processes, such as high purity oxygen, have much higher DO levels and are indicative of their process.

Water Treatment Sample Question

What is the National Primary Drinking Water Regulation removal requirement for Legionella?

A. 99% removal
B. 99.9% removal
C. 99.99% removal
No removal limit required 

Answer: The correct answer is D. Legionella bacteria is the cause of legionnaires disease, which can be deadly to individuals with an immune system disorder. Legionella bacteria are naturally occurring in water and can multiply rapidly in water heating systems where chlorine for disinfection is not present. However, the Environmental Protection Agency believes Legionella will be controlled if the required Surface Water Treatment Rule removal/inactivation of Giardia and viruses are met. 

About the authors: Rick Lallish is the Water Pollution Control program director at the Environmental Resources Training Center (ERTC) of Southern Illinois University Edwardsville. He provides training for entry-level operators in the wastewater field and operators throughout the state looking to further their education. Lallish was also named the 2017 Illinois Operator of the Year and 2018 president of the Illinois Association of Water Pollution Control Operators.

Drew Hoelscher is the program director of drinking water operations at the Environmental Resources Training Center in Edwardsville, Illinois.


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